Hochtief has been against the deal, saying it does not add any value for its shareholders, and hundreds of its employees took to the streets in Berlin in October in protest.

Hochtief, which is based in Essen, reported a net profit of euro88.1 million ($115.82 million) in this year's first half, up 2.1 percent from a year earlier.

The company is involved in large construction projects such as office buildings and schools, and owns the construction subsidiaries Turner and Flatiron in the U.S. and Leighton in Australia.

A consortium led by Hochtief's U.S. subsidiary is building a new access road to San Francisco's Golden Gate Bridge in a $1 billion deal.

Hochtief, which has some 70,000 employees, also has stakes in airports, including Duesseldorf, Germany; Tirana, Albania; and Sydney, Australia.

ACS, based in Madrid, is traditionally associated with large public works projects, but diversified in recent years as Spain's economy weakened, buying stakes in electrical utility Iberdrola and toll road operator Abertis.